\section{Zariski Topology}
\begin{definition}[Zariski Covering]\label{ZarCov}
Let $\bcU=\{f_i:U_i\rightarrow X\}_{i\in I}$ be a family of open immersions in $\Sch$, if for every point $x\in X$, there exist $i\in I$, and $u\in U_i$, over $x$, i.e. $f_i(u_i)=x$, we call the family $\bcU$ Zariski covering.
\end{definition}
\begin{lemma}
\noindent The collection of morphisms in $\Sch$ that satisfies the condition of definition \ref{ZarCov} forms a pre-topology on $\Sch$, in the sense of Grothendieck.
\end{lemma}